How Planets Are Made

Comets are so much more than clusters of rock floating in the stratosphere. In fact, scientists believe comets—some as old as 4.5 billion years—containing primordial materials that date back to the beginning of the solar system, might help us unlock the secret of how the planets were made.

Comets That Came Before

Until now, only four comets have been "visited" by U.S. spacecraft: Giacobini-Zinner, Borrelli, Wild 2, and Tempel 1. The first two comet visits yielded some worthwhile photos, but it wasn't until the third visitation that things really started to get interesting.

First was Wild 2. On Jan. 2, 2004, the Stardust spacecraft collected samples as it flew through the tail of dust and gas of the comet and parachuted these samples to Earth two years later. Scientists were thrilled to discover evidence of amino acids, the building blocks of proteins, the stepping stones to life.

Next, the Deep Impact spacecraft visited the comet Tempel 1 in 2005, releasing an 800-pound piece of equipment that crashed into the comet as it passed by. The collision allowed scientists to identify some of the minerals beneath the surface of Tempel 1.

Comet Flyby

Then, on Nov. 4, 2010, the first stunning images of Hartley 2 were beamed back to Earth as the Deep Impact spacecraft (the same craft that visited Tempel I) came within 435 miles of the comet. Michael F. A'Hearn, the mission's principal investigator, said scientists would compare Hartley 2 with Tempel 1 to better understand all comets.

Scientists believe that the three-quarters of a mile wide, peanut-shaped comet was originally two space objects that came together and stuck. What looked like bright jets shooting from the surface turned out to be hundreds of pounds of fluffy ice chunks that the comet was spewing every second. "To me, this whole thing looks like a snow globe that you've simply just shaken," Peter H. Schultz, a professor working on the mission, said during a news conference.

The novel discovery was that the ice sprays were not caused by forced water vapor heated by the Sun, but rather by jets of carbon dioxide. This marked the first time that such carbon dioxide jets had been observed in a comet. And, you guessed it, the frozen carbon dioxide is most certainly primordial, dating from the beginnings of our solar system.